Not sure if this has been discussed before, but I think it would be very useful.

If we had deterministic addresses made by a huge pass-phrase, plus a JavaScript library that can create and sign transactions, we could potentially have web wallets that don't hold any coins and instead just check your balance and post transactions to the network. The sites themselves could even charge an additional transaction fee, refusing to forward transactions that don't pay up. It of course wouldn't be completely secure, but IMO it would be a lot more secure than a centralized banking system like MyBitcoin.

Is this feasible/desirable? Has any work been done on this idea or one like it?